This invention pertains to the noninvasive measurement of respiratory muscular effort such as might be useful, for example, in control of the triggering of a mechanical ventilator.
It is useful to be able to measure respiratory effort for a number of purposes, including determining when and by how much a subject is attempting to breathe; detecting a central apnea or hypopnea, increased upper airway resistance or obstructive sleep apnea; and triggering of mechanical ventilators.
The prior art includes the direct invasive measurement of pleural pressure, for example, by using a pressure sensor in the pleural cavity or esophagus, or using an esophageal balloon connected to a pressure transducer; or measurement of transdiaphragmatic pressure difference as an indication of diaphragmatic contraction using catheters, balloons, or sensors in the esophagus and stomach. These methods are very invasive and uncomfortable, and not suitable for prolonged use or use during natural sleep.
Other prior art includes measurement of the electrical activation of various respiratory muscles, for example, the diaphragm, intercostal muscles, or various respiratory accessory muscles such as the alae nasi muscles.
Additional prior art uses various detectors of ribcage or abdominal movement, including inductance pneumograms, strain gauges, magnetometers, pneumatic belts, and the like.
The prior art most directly relevant to the present invention noninvasively estimates changes in pleural pressure by measuring the resultant deformation of the suprasternal notch. One known method teaches directly measuring the deformation of the skin of the suprasternal notch with a rigid mechanical probe in contact with the skin, and connected to a strain gauge or similar sensor of movement anchored to the sternum. U.S. Pat. No. 4,180,059 in the name of Tiep describes using a rigid mechanical probe in contact with the skin. Deformation of the skin by changing pleural pressure causes movement of the rigid probe, and thereby activation of the strain gauge or other position sensor. A disadvantage of this method is the need to have the probe actually touching the skin, which makes the device both mechanically difficult to position, and annoying to the subject.
A second known method uses a strip of piezo-electric material such as PVDF glued to the skin of the suprasternal notch. Deformation of the skin causes deformation of the PVDF, resulting in an electric charge developing between upper and lower surfaces which may then be measured. This method does not give good signals at low effort, and the finite input impedance of the charge amplifier results in a short time constant. It also requires the strip to be glued into the suprasternal notch, which is inconvenient for longterm use.
The invention teaches the use of an optical sensor to measure movement of the skin of the suprasternal notch, this movement being a measure of respiratory effort. The invention further teaches the use of a trough detector with a time constant long compared with a breath but short compared with the interval between changes in body position, to detect the optical signal corresponding to zero effort, and to subtract the trough signal from the optical signal to produce a self-zeroing effort signal which automatically compensates for changes in body position. The invention yet further teaches co-mounting of the sensor with any required electronics, and sending the effort signal to a radio receiver by telemetry. Finally, the invention teaches the use of a soft, low irritant adhesive material which may be left in place on the skin for several days, the sensor being placed and removed as often as desired, without injuring the skin.